Preface (M. Hargittai, I. Hargittai). Molecular geometry of "ionic" molecules: a ligand close-packing model (R.J. Gillespie, E.A. Robinson). The terminal alkynes: a versatile model for weak directional interactions in crystals (T. Steiner). Hydrogen bonding systems in acid metal sulfates and selenates (E. Kemnitz, S.I. Troyanov). A crystallographic structure refinement approach using abinitio quality additive fuzzy density fragments (P.G. Mezey). Novel inclusion compounds with urea/thiourea/selenoureaanion host lattices (T.C.W. Mak, Q. Li). Roles of zinc and magnesium ions in enzymes (A. Kaufman Katz, J.P. Glusker). The electronic spectra of ethane and ethylene (C. Sandorfy). Formation of (E,E) and (Z,Z) muconic acid in metabolism of benzene: possible roles of putative 2,3-epoxyoxepins and probes for their detection (A. Greenberg). Some relationships between molecular structure and thermochemistry: part 2 (J.F. Libeman, S.W. Slayden). Index.